1. Field of the Invention
This invention relates to photography, and more particularly, it relates to photographic processes performed in ambient light and to photographic products useful in such processes.
2. Description of the Prior Art
A number of diffusion transfer processes for producing photographic images in both black-and-white and in color are now well known. Of particular interest are diffusion transfer processes wherein the image-receiving layer carrying the transfer image is not separated from the developed photosensitive layer(s) after processing but both components are retained together as a permanent laminate. Included as part of the laminate is a layer of a light-reflecting material, preferably titanium dioxide, positioned between the image-carrying layer and the developed photosensitive layer(s). The light-reflecting layer separating the image-carrying and photosensitive components provides a white background for the transfer image and masks the developed photosensitive layer(s). In addition to these layers, the laminate usually includes dimensionally stable outer layers or supports, at least one of which is transparent so that the resulting transfer image may be viewed by reflection against the background provided by the light-reflecting layer. Diffusion transfer processes for forming images viewable without separation of the photosensitive and image-receiving components and film units useful in such processes are described, for example, in U.S. Pat. Nos. 3,415,644, 3,415,645 and 3,415,646 issued Dec. 10, 1968 to Edwin H. Land.
U.S. Pat. No. 3,647,437 issued Mar. 7, 1972 to Edwin H. Land also is concerned with diffusion transfer processes wherein the resulting photograph comprises the developed photosensitive layer(s) retained with the image-receiving layer as part of a permanent laminate. In the processes disclosed in this patent, a photographic film unit comprising a photosensitive element is developed in ambient light but further undesired exposure during processing is prevented by a light-absorbing material or optical filter agent which is retained in the processed film unit. In a preferred embodiment, the optical filter agent is a pH-sensitive dye, i.e., a dye possessing spectral absorption characteristics that are reversibly alterable in response to changes in environmental pH and particularly, a pH-sensitive dye having a colored or light-absorbing form above a given alkaline pH and a colorless or non-light-absorbing form below said pH. Examples of pH-sensitive dyes found particularly useful as light-absorbing optical filter agents are the phthaleins, i.e., the phthalide and naphthalide dyes derived from indoles disclosed in U.S. Pat. No. 3,702,244 issued Nov. 7, 1972 to Stanley M. Bloom, Alan L. Borror, Paul S. Huyffer and Paul T. MacGregor and the phthalide and naphthalide dyes derived from 1-naphthols disclosed in U.S. Pat. No. 3,702,245 issued Nov. 7, 1972 to Myron S. Simon and David P. Waller. As discussed in these and other patents, a combination of the indole and 1-naphthol dyes generally is used where it is desired to provide protection from post-exposure fogging throughout the visible spectrum.
In a particularly useful embodiment disclosed in said U.S. Pat. No. 3,647,437, the film unit is of the type described in aforementioned U.S. Pat. No. 3,415,644 and comprises a first sheet-like component comprising an opaque support carrying a silver halide emulsion layer(s) and a second sheet-like component comprising a transparent support carrying an image-receiving layer which are in fixed relationship prior to exposure, which relationship is maintained after processing. After photoexposure through said transparent support, an aqueous alkaline processing composition is distributed in a thin layer between said components. The processing composition contains a light-reflecting pigment and at least one light-absorbing optical filter agent, such as, one of the aforementioned phthalein dyes which is in its colored form at the initial pH of said aqueous alkaline processing composition and which, after at least the initial stages of processing, is converted to its colorless form by reducing the environmental pH, for example, by including an acid-reacting layer as part of the film unit. The concentrations of the light-reflecting pigment and light-absorbing optical filter agent required to provide adequate protection of the photosensitive layer(s) will vary with the process being performed and the anticipated conditions, e.g., light intensity, dark time, etc. Preferably, the concentrations of these materials are such that the processing composition layer containing the pigment and filter agent will have a transmission density of at least about 6 but a reflection density not greater than about 1.
U.S. Pat. No. 4,298,674 issued Nov. 3, 1981 to Edwin H. Land, Leon D. Cerankowski and Neil Mattucci discloses diffusion transfer processes wherein the optical filter agent contained in the pigmented processing composition layer is decolorized adjacent the interface of the processing composition layer and image-receiving component in order to render the interface or image viewing background substantially "white" initially and throughout processing. Since this decolorization is limited to a small concentration of optical filter agent adjacent said interface, the transmission density of the processing composition layer is not reduced to any significant extent, and thus, adequate protection from post-exposure fogging is provided until such time as it is desired to decolorize the remaining optical filter agent.
As described in said U.S. Pat. No. 4,298,674, the decolorization of the optical filter agent at said interface is achieved by employing film units wherein the image-receiving component carries a layer containing a substantially non-diffusible agent adapted to reduce the light-absorbing ability of the optical filter agent immediately adjacent the interface between said layer and the pigmented processing composition layer without reducing the light absorbing ability of the optical filter within said processing composition layer. In a preferred embodiment, the decolorization agent is a neutral polymeric material, such as a polyvinyl pyrrolidone or a polyether, which material is believed to effect decolorization in aqueous alkali by forming a complex with a salt of the pH-sensitive optical filter agent formed with the cation of said alkali, e.g., K.sup.+, wherein the complex exhibits a higher apparent pKa than the pH-sensitive dye. Because of the increase in apparent pKa, decolorization occurs without a reduction in pH. The polymeric ethers in particular exhibit a propensity for binding many cations, thus becoming a "super cation" which changes the apparent pKa of the pH-sensitive phthalein dye.